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Titanium dioxide

chemical compound

Titanium dioxide, also known as titanium(IV) oxide or titania /taɪˈteɪniə/, is the inorganic compound with the chemical formula TiO2. When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insoluble to water, although mineral forms can appear black. As a pigment, it has a wide range of applications, including paint, sunscreen, and food coloring. When used as a food coloring, it has E number E171. World production in 2014 exceeded 9 million tons. It has been estimated that titanium dioxide is used in two-thirds of all pigments, and pigments based on the oxide have been valued at a price of $13.2 billion. Nanoscopic scale titanium dioxide, particularly in the anatase form, exhibits photocatalytic activity under ultraviolet (UV) irradiation. In addition to its use as a pigment, titanium dioxide can be added to paints, cements, windows, tiles, or other products for its sterilizing, deodorizing, and anti-fouling properties, and is used as a hydrolysis catalyst. It is also used in dye-sensitized solar cells (Grätzel cells).

Titanium(IV) oxide

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Carbon Black, Titanium Dioxide, and Talc (2010)

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IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Vol. 93, World Health Organization International Agency for Research on Cancer. (Views and Expert Opinions of an IARC Working Group which met in Lyon, France, Feb 7-14, 2006.)
  • Titanium dioxide was first produced commercially in 1923, primarily for pigment production. Relatively small quantities are used for non-pigmentary purposes.
    • Summary of Data Reported, p. 272.
  • Titanium dioxide is obtained from a variety of ores that contain ilmenite [FeTiO3], rutile [TiO2, but may contain up to 10% iron and significant amounts of niobium and tantalum], anatase [TiO2] and leucoxene, which are mined... throughout the world.
    • Summary of Data Reported, p. 272.
  • Most titanium dioxide pigment is produced from titanium mineral concentrates by the chloride or sulfate process, either as the rutile or as the anatase form.
    • Summary of Data Reported, p. 272.
  • Exposure levels [to titanium dioxide]... are assumed to be low... with the possible exception of workers who handle large quanities... No significant exposure... is thought to occur during the use of products in which titanium dioxide is bound to other materials, such as paints.
    • Summary of Data Reported, p. 272.
  • There is inadequate evidence in humans for the carcinogenicity of titanium dioxide. ...There is sufficient evidence in experimental animals for the carcinogenicity of titanium dioxide. ...Titanium dioxide is possibly carcinogenic to humans.
    • Evaluation and Rationale, p. 275.
  • In two studies of rats that inhaled titanium dioxide, one observed an excess incidence of lung tumours... No increases were observed among mice and hamsters exposed intratracheally. Other studies that used different routes of administration did not observe excesses in tumour incidence.
    • Evaluation and Rationale, p. 275.
  • The Working Group considered the body of evidence regarding the pathways and mechanisms by which titanium dioxide and other poorly soluble particles may cause cancer. ...[T]he available mechanistic evidence for titanium dioxide was not strong enough to warrant classification other than Group 2B.

"Titanium dioxide in everyday life; is it safe?" (Nov 2011)

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Matej Skocaj, Metka Filipic, Jana Petkovic and Sasa Novak, Radiological and Oncology, Vol. 45, Issue 4, pp. 227 - 247.
  • TiO2 has been classified in humans and animals as biologically inert... and is widely considered to be a "natural" material...
  • The annual worldwide production of titania powder in 2005 has been estimated to be around 5 million tons... The proportion of nano-sized titania is estimated to have been approximately... 10% by 2015... with an exponential increase over the past decade.
    • Introduction, p. 227.
  • TiO2 powders have begun to appear in many applications, mainly due to their ability to confer whiteness and opacity on... products, such as paints, papers and cosmetics.
    • Introduction, p. 227.
  • [I]ts light-scattering properties and very high refractive index, which mean that relatively low levels of the pigment are required to achieve a white, opaque coating.
    • Introduction, p. 227.
  • Numerous technological improvements, based on nano-sized TiO2... enable its use for antifogging and self-cleaning coatings on glass, for building facades, in confectionary, in the plastics industry... [etc.]
    • Introduction, pp. 227-228.
  • [I]t is included in the Food and Drug Administration (FDA) Inactive Ingredients Guide for dental paste, oral capsules, suspensions, tablets, dermal preparations and in non-parenteral medicines.

"Heterogeneous Photocatalysis: Recent Advances and Applications" (March 1, 2013)

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Alex Omo Ibhadon, Paul Fitzpatrick, Catalysts, 2013, 3(1), pp. 189-218.
  • Semiconductor heterogeneous photocatalysis... is a versatile, low-cost and environmentally benign treatment technology for a host of pollutants. ...photocatalysis demands that the pollutant, the catalyst and source of illumination are in close proximity or contact ...The ability of advanced oxidation technology to remove low levels of persistent organic pollutants as well as microorganisms in water has been widely demonstrated and... the technology is now being commercialized... This review considers recent developments... for the treatment of low-level concentrations of pollutants in water and air using titanium dioxide as a "model" semiconductor.
    • Abstract
  • In recent years... use of semiconductor materials as photocatalysts for the removal of... organic and inorganic species from aqueous or gas phase systems in environmental clean-up, drinking water treatment, industrial and health applications... is because of the ability of TiO2 to oxidize organic and inorganic substrates in air and water through redox processes. ...TiO2 has ...emerged as one of the most fascinating materials in... catalysis... engaging... physical chemists, physicists, material scientists and engineers in exploring distinctive semiconducting and catalytic properties.
    • Introduction
  • Inertness to chemical environment and long-term photostability has made TiO2 an important material... in commercial products... from drugs to foods, cosmetics to catalysts, paints to pharmaceuticals, and sunscreens to solar cells in which TiO2 is used as a desiccant, brightener, or reactive mediator... The catalyst itself is unchanged during the process and no consumable chemicals are required. This results in... savings and simpler operation of... equipment...
  • Introduction
  • The U.S. Food and Drug Administration permits up to 1% TiO2 as an inactive ingredient in food products. While there are no known health effects... a recent study found that 3–6 year old children are the most affected group of people that consume TiO2 particles from food products.
  • Introduction
 
Schematic of semiconductor excitation by band gap illumination leading to the creation of “electrons” in the conduction band and “holes” in the valance band.
  • TiO2 response to UV light has... led to photocatalysis research... also to... investigation of TiO2 superhydrophilicity... use in environmental remediation and solar fuel production. Bandgap excitation of TiO2 causes charge separation followed by scavenging of electrons and holes by surface adsorbed species...
  • Introduction
  • Visible photocatalysis can... be utilized by doping TiO2 with a sensitizing dye or short bandgap semiconductors. ...[T]here remain some challenges with respect to extending the photocatalytic response of TiO2 in the visible region... an area that merits fundamental research.
  • Introduction
  • [H]eterogeneous photocatalysis using semiconductors such as titanium dioxide is... efficient... because as the... process gradually breaks down the contaminant molecules, no residue of the original material remains and therefore no sludge requiring disposal... is produced.
  • Introduction
  • In order to activate the degradation process, pure TiO2 requires photo-excitation with light at wavelengths exceeding the band gap... that is, wavelengths of <387 nm. ...[C]ommercially available photocatalytic water treatment plant using artificial UV-light... can be considered as a developed market.
  • Introduction
  • UV in natural sunlight represents only 5%–8% of the solar spectrum at sea level and this provides a limitation and the requirement of artificial illumination of the catalysts... The need for more stable, efficient catalysts which can be activated by natural sunlight is now the goal of researchers. ...[P]recise control of the stoichiometry of the catalytic metal oxides and mixed metal oxides, addition of dopants, particle shape and pore topology are all critical factors for catalyst developers.
  • Introduction
  • Photocatalysts that have high activity using wavelengths of light in the visible spectrum (380 nm < λ < 500 nm) ...has been achieved by altering the band gap... Doping of TiO2 with transition metal ions (for example, V, Cr, Mn, Fe and Ni) as well as with Ag, Au and Ru, have been demonstrated to red-shift the TiO2 absorption band from the UV into the visible region...
  • Introduction
  • With respect to... TiO2, photocatalytic reactions are initiated by the absorption of illumination with energy equal to or greater than the band gap of the semiconductor. This produces electron-hole (e−/h+) pairs as in...
    TiO2+hv=e-cb(TiO2)+h+vb(TiO2)
    where cb is the conduction band and vb is the valence band. [A]s a result... the TiO2 particle can behave either as an electron donor or acceptor for molecules in contact... The electron and hole can recombine... On the other hand, they can participate in redox reactions with adsorbed species as the valence band hole is strongly oxidizing while the conduction band electron is strongly reducing. ...[T]he excited electron and the hole can participate in redox reactions with water, hydroxide ion (OH−), organic compounds or oxygen leading to mineralization of the pollutant. ...[R]eactions with water are predominant since the water molecules are more abundant ...[O]xidation of water or OH- by the hole produces the hydroxyl radical (·OH), a powerful oxidant. ...OH radicals are able to rapidly attack pollutants on the semiconductor surface and... are the most important radicals formed in TiO2 photocatalysis. An important reaction of the conduction band electron is reduction of adsorbed oxygen to oxygen radicals... [T]his prevents the electron from recombining with the hole and results in an accumulation of oxygen radicals... also... degrading contaminants in solution...
    • Mechanism of Semiconductor Catalysis

"Titanium Dioxide-Coated Zinc Oxide Nanorods..."] (Aug 14, 2020)

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"as an Efficient Photoelectrode in Dye-Sensitized Solar Cells" Zhang Q, Hou S, Li C. Nanomaterials. 2020; 10(8):1598.
  • Since Grätzel et al. developed the titanium dioxide (TiO2)-based dye-sensitized solar cell (DSSC) in 1991... the DSSC has emerged as a promising photovoltaic device, due to its promising power conversion efficiency (PCE), low fabrication cost, and low toxicity...
    • Introduction
  • Hitherto, it has been reported that TiO2-based DSSCs achieved a notable PCE of over 14%... However, further improvements in PCE are difficult to achieve due to some disadvantages in current TiO2-based DSSCs, such as the low carrier transportation rate of TiO2 resulting from its low electron mobility, as well as the difficulty in fabricating TiO2 nanostructures with a large surface-to-volume ratio...
    • Introduction

"Commission Regulation (EU) 2022/63..." (Jan 14, 2022)

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"amending Annexes II and III to Regulation (EC) No 1333/2008 of the European Parliament and of the Council as regards the food additive titanium dioxide (E 171)" (Text with EEA relevance)
  • (3) Titanium dioxide (E 171) is a substance authorised as a colour in certain foods, in accordance with Annex II to Regulation (EC) No 1333/2008.
  • (11) On 6 May 2021, the Authority published a scientific opinion on the safety assessment of titanium dioxide (E 171) as a food additive... [F]ollowing the 2018 Guidance on nanotechnology, the opinion also takes into account, in addition to all new relevant data, the data on the potential genotoxicity of titanium dioxide nanoparticles published before 2016, that had not previously been identified as relevant for the 2016 re-evaluation. In its opinion the Authority indicated that, based on all the evidence available, a concern for genotoxicity could not be ruled out, and given the many uncertainties, it concluded that titanium dioxide (E 171) can no longer be considered safe when used as a food additive. ...
  • (13) However, given that the Authority did not identify an immediate health concern linked to titanium dioxide (E 171) used as a food additive and in order to allow for a smooth transition, it is appropriate that foods that contain titanium dioxide (E 171) used in accordance with the rules applicable before the date of entry into force of this Regulation may be placed on the market until six months after that date. Those foods may then continue to be marketed until their date of minimum durability or ‘use by’ date.

"Inactivation and Degradation of Influenza A Virus..." (Oct 23, 2022)

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"on the Surface of Photoactive Self-Cleaning Cotton Fabric Functionalized with Nanocrystalline" by Selishchev, D.; Stepanov, G.; Sergeeva, M.; Solovyeva, M.; Zhuravlev, E.; Komissarov, A.; Richter, V.; Kozlov, D. Catalysts 2022, 12, 1298. https://doi.org/10.3390/catal12111298
  • Chemical modification of cotton-rich fabrics with TiO2 nanoparticles results in photoactive self-cleaning textiles, which can provide, under UV or solar radiation, complete oxidation of low-molecular compounds, degradation of supramolecular structures, and inactivation of microorganisms due to the photocatalytic effect. ...Fast inactivation of influenza virus [based on the example of influenza A (H1N1) virus] occurs on the irradiated surface of photoactive fabric due to adsorption and photocatalytic degradation. The TiO2 component in the prepared fabric increases the adsorption effect compared to initial cotton due to a high specific area of TiO2 nanocrystallites. ...The results of this study underline the potential of photoactive self-cleaning fabrics for application in air purification systems and personal protective clothes to provide permanent protection... against harmful chemical and biological pollutants.
    • Abstract
  • Photoinduced self-cleaning and antimicrobial properties are observed for fabrics modified with photocatalysts, especially with TiO2 of anatase structure because it is the most active and stable photocatalyst for oxidative degradation in oxygen-contained medium... TiO2 is commonly used in other photocatalytic processes, including photocatalytic hydrogen production... CO2 reduction... and photoelectrochemical conversion... Nanocrystalline TiO2 can be anchored on the external surface of nonporous and porous supports (e.g., glasses... adsorbents... polymeric fibers... and fabrics... to prepare composites with enhanced properties or functional materials.
    • Introduction
  • Fabrics functionalized with nanocrystalline photocatalysts can completely decompose low-molecular compounds due to formation of reactive oxygen species under corresponding radiation. In the case of TiO2, OH-radicals are regarded as the main species that provides oxidation reactions. Degradation of formaldehyde... acetone... phenol... and gaseous ammonia... over photoactive fabrics was previously... described. High-molecular organic compounds can also be degraded...
    • Introduction
  • The conventional method to evaluate the activity of photoactive fabrics is discoloration of spots under radiation. Organic dyes... coffee... wine... juice... make-up, and sweat... were used... Complete discoloration of spots can be achieved on the surface of photoactive fabrics after long-term irradiation.
    • Introduction
  • [T]he photoactive component of self-cleaning fabrics can lead to degradation of membranes and cell walls of various microorganisms (e.g., protozoa, bacteria, endospores, fungi, and algae), resulting in their death... Under irradiation, faster inactivation of bacteria typically occurs on the surface of photoactive fabrics compared to the initial textile substrates. ... Typically, degradation of fungi is substantially slower compared to degradation of bacteria due to fungi having a thicker cell wall...
  • TiO2-based photocatalysts are known to be able to inactivate viruses under UV irradiation. ..irradiated photocatalyst promotes destruction of virions due to formation of reactive oxygen species.

See also

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